Many of the regular pet shampoos and conditioners use detergents and other chemicals that strip the coat and skin. Some ad conditioners to help replace the natural oils their product has stripped from the skin. It's like over processing your hair and then conditioning it to make up for the damage that has been done.
I would use an all natural product. A shampoo with a separate conditioner. An additional spray conditioner for between baths would also be advisable.
I use the Yorkie Splash and Shine shampoo and conditioner along with the conditioning spray.
Yorkie Splash and Yorkie Shine | Yorkie Shampoo and Yorkie Conditioner
There are other all natural products out there as well. You are dealing with a puppy coat that is in transition. Some of the puppy coat is coming out as the new coat is coming in and that causes knots or mats. Use a metal grooming comb everyday before brushing giving special attention to under the legs, behind the ears and the rear areas and belly. Long coated breeds require special attention daily. Even if you keep him cut short the coat still needs daily combing to keep it in tip top shape.